In the reduction of iron ores to metallic iron, increased interest has been shown of late in systems for the direct reduction of iron ore, in the form of pellets, granules, particles and briquettes, in shaft-type furnaces. In such systems, a charge of iron ore is caused to descend in the shaft furnace in counterflow to a rising stream of reducing gas, generally consisting largely of carbon monoxide and hydrogen. The reducing gas may be produced from process hydrocarbons, consisting mainly or especially of methane, by reformation of the gaseous hydrocarbons with gases containing oxygen, e.g., air, furnace-exhaust gas, etc., in reformers which may operate alternately to provide a continuous flow of the reducing gas.
The product is metallized iron-containing sponge or sponge iron which may then be smelted elsewhere and can be used as the starting material for a steel-making or other refining process.
When the reformer is not being used for the production of the reducing gas it may be heated by combustion or regenerative techniques. In the next cycle or phase, the heated generator is used to produce the reducing gas while the heat-depleted regenerator is heated as described above. The term "reformer" as used herein is thus used to refer to Cowper-like vessels provided with heat storage masses in the form of regeneratively heated lattice works of ceramic bricks or the like.
In a conventional system, wherein the reformers are constituted by simple Cowpers traversed alternately by the process hydrocarbons and gasification medium, the reformers feed the reduction furnace directly. The reformation reactions can include the following: EQU CH.sub.4 + CO.sub.2 = 2CO + 2H.sub.2 EQU 2ch.sub.4 + o.sub.2 = 2co + 4h.sub.2 EQU ch.sub.4 + h.sub.2 o = co + 3h.sub.2.
these reactions, among others, are generally endothermic, deriving the heat necessary to drive the reactions from the previously heated latticework of the reformers. Thus the temperature of the product, i.e. the reducing gas, falls from the start to the end of the reformation cycle and the temperature of the reducing gas introduced into the reducing furnace may fluctuate by several hundreds of degrees centrigrade. While such fluctuations have generally been tolerated heretofore, they disadvantageously affect the reducing process and especially the rate at which the iron ore is reduced. The problem is significant whether the reforming or gasification medium is oxygen or the furnace waste gases which are rich in carbon dioxide and water vapor.